(1) Field of the Invention
This invention relates to the method for the preparation of base/clay composites for the removal of SO.sub.x, (sulfur dioxide and sulfur trioxide), from sulfur-containing gas streams, particularly flue gas from coal burning power plants, and to the process of using these sorbents for the removal of SO.sub.x. The composites contain a base component selected from alkaline earth metal hydroxides and/or carbonates and a second metal oxide or oxide precursor, preferably selected from transition metal ions, capable of promoting the oxidation of sulfur dioxide to sulfur trioxide, and a dispersing agent selected from the smectite clays to disperse the base and catalyst.
(2) Prior Art
Coal represents the large resource of fossil energy in the world, today. For example, it has been estimated that the known coal reserves in the U.S. alone could supply sufficient energy for domestic consumption for several hundred years. One of the major problem of using coal, as energy source is the presence of sulfur. As a result, in fossil-fuel-fired power plants, the sulfur content of the feed coal is oxidized during combustion to sulfur oxides (SO.sub.2 and SO.sub.3, commonly referred to as "SO.sub.x "), which are released through stacks to the atmosphere. Analyses of flue gas produced by power plants burning coal before desulfurization, show 0.5%-0.2% SO.sub.2 and about 0.005% SO.sub.3. One of the most serious environmental problems associated with such sulfur emissions is the generation of sulfuric acid, resulting in so called "acid rain".
Control of sulfur oxide emissions is mandated by the United States Environmental Protection Agency (EPA), and in 1970, the Clean Air Act Amendments were adopted by U.S. Government for this purpose This legislation provided for enforcement, by EPA, of SO.sub.x emissions limits for power plants constructed or modified after Aug. 17, 1971. This Act spurred extensive flue gas desulfurization (FGD) research and various studies are under way to develop methods for SO.sub.x removal from flue gas streams. As of January 1984, calcium based, wet, throwaway systems (including lime, limestone, and alkaline-ash systems) accounted for 84 percent of existing and planned FGD capacity The Clean Air Act was amended in 1977 and very recently in 1990, to require further control of SO.sub.x emissions. The Clean Air Act of 1990 requires, among other things, that coal-fired power plants cut sulfur dioxide emissions by half, or about 9 million tons annually, in the next decade. Increasing federal regulations and the high cost to construct and operate existing wet FGD units have encouraged continued research on new or modified flue gas cleanup processes.
Controlling the emissions of SO.sub.x from power plants is a world-wide problem and research into its control is a global effort. Formation of SO.sub.x in combustion processes can be reduced by modifying the burner design and combustion system, by changing the operating conditions and by using fuels with lower sulfur contents. The most popular and inexpensive method of reducing SO.sub.x emission is the addition of reactive dry sorbents with the fuel. Accordingly at present, SO.sub.x removal is most often accomplished by using lime (CaO), lime stone (CaCO.sub.3) or hydrated lime (Ca(OH).sub.2) due to cost effectiveness and available quantities. For example, in U.S. Pat. No. 4,731,233 to Thompson and Nuzio, describe the use of these calcium based sorbents to reduce SO.sub.x emissions from flue gas streams.
In typical coal-fired power plants the ground sorbent, for example lime or limestone, is added into boilers along with coal or sprayed into towers as a slurry to contact the flue gas. The SO.sub.2 reacts with calcium hydroxide to form a calcium sulfite slurry which is then partially oxidized with air to calcium sulfate. In this way the sulfur oxides are retained as harmless solid compounds which can be removed from the stack gas by electrostatic precipitation or other standard methods. Such a process is potentially attractive for retro-fitting existing power plants since no major structural alterations are required.
Although calcium based systems are the major source of SO.sub.x control, they are not without problems. Agglomeration of particles can be a serious problem that results in a less than optimal conversion to CaSO.sub.x, (CaSO.sub.3 and CaSO.sub.4). The activity of the calcium species decreases as its particle size increases. Also CaSO.sub.x occupies more volume than CaO, the common active species. Therefore, an increase in volume occurs as the reaction proceeds, which causes a loss in the original porous character of the CaO. This results in a blockage of SO.sub.x and O.sub.2 to the active CaO centers (Gullett, B. K. and Blom, J. A., React. Solids, 3 337 (1987); Gullett, B. K., Blom, J. A. and Cunningham, R. T., React. Solids, 6 263 (1988); Chang, E. Y. and Thodes, G., AIChE J., 30 450 (1984); Thibault, J. D., Steward, F. R. and Ruthven, D. M., Can. J. Chem. Eng., 60 796 (1982)). Hence in the relatively short contact time available, only a small fraction of the sorbent reacts. In principle the problem of low utilization of the sorbents may be solved by reducing the particle size, but in practice, the particle size required for a reasonable level of utilization may be too small to achieve economically by conventional grinding or fragmentation methods.
Prior art in this field considered several methods to develop reactive limestone, lime or hydrated lime as a precursor for the active CaO species or has used Ca(OH).sub.2 as the active species. Generally, the active species has been used as a bulk phase and not as a dispersed species (Chang, J. C. S. and Kaplan, N., Envir. Prog., 3 267 (1984); Gullett, B. K., Blom, J. A. and Cunningham, R. T., React. Solids, 6 263 (1988); Chang, E. Y. and Thodes, G., AIChE J., 30 450 (1984); Fuller El L. and Yoos, T. R., Langmuir, 3 753 (1987)).
Recent work has concentrated on the addition of fly ash to Ca(OH).sub.2 to enhance its activity in SO.sub.x control (Jozewicz, W. and Rochelle, G. T., Envir. Prog, 5 219 (1986); Jozewicz, W., Chang, J. C. S., Sedman, C. B. and Brna, T. G., JAPCA, 38 796 (1988); Jozewicz, W., Chang, J. C. S., Sedman, C. B. and Brna, T. G., React. Solids, 6 243 (1988); Jozewicz, W., Chang, J. C. S., Sedman, C. B. and Brna, T. G., EPA/600/d-87/095, (NTIS PB87-175857/AS); Jozewicz, W., Chang, J. C. S., Sedman, C. B. and Brna, T. G., EPA/600/D-87/135, (NTIS PB87-182663). The fly ash is a siliceous material and it is believed that formation of various calcium silicates occur. Several diatomaceous earths, including clays such as montmorillonites and kaolins have also been identified as suitable silicates to obtain reactive silica for the formation of calcium silicates(Jozewicz, W., Chang, J. C. S., Sedman, C. B. and Brna, T. G., React. Solids, 6 243 (1988)). In coal burning power plants these sorbents were claimed to be suitable for the cold-side SO.sub.x removal where sorbents are added to the duct-work down stream of the boiler. The enhanced reactivity observed in these sorbents were attributed to the presence of different forms of calcium silicates. These results have been also disclosed in U.S. Pat. Nos. 4,804,521 and 4,931,264.
Voss in U.S. Pat. No. 4,387,653 describes limestone-based sorbent agglomerates that are suitable for the removal of sulfur-containing compounds such as hydrogen sulfide and sulfur oxides arising from combustion of organic fuels such as coal in fluidized bed units. The agglomerates are prepared using the conventional aggregation technology by mechanically crushing the naturally occurring limestone into grains of fine particle size and then binding the powdered limestone grains together with a binder. Different types of clays including, attapulgites, and other smectite clays have been used as suitable binders. Use of binders facilitated the formation of agglomerates with certain particle size (-14 to +32 mesh) that are ideal for the proper fluidization of particles in the fluidized bed combustion of coal.
U.S. Pat. No. 4,830,840 describes a sorbent composition containing an alkaline earth metal, aluminum-containing spinel/clay compositions for SO.sub.x capture. Spinels are mixed metal oxides that posses chemical compositions different from simple alkaline earth metal oxides or their carbonates and hydroxides. Furthermore, this U.S. patent describes the use of kaolin clay as the matrix material, which has different properties than smectite type clays. The major distinction of smectite type clays compared to kaolin type clays is the ability of smectites to swell in water. These swelling properties are in part responsible for the higher reactivity of the sorbents of this invention.
U.S. Pat. No. 4,952,382 to van Broekhoven has recently disclosed a catalyst composition suitable for the refining of heavy sulfur- and metal-containing petroleum feeds. An "anionic clay" component present in the catalyst serves as the sorbent for the removal of SO.sub.x from feed gas in fludized catalytic cracking units. Anionic clays are primarily synthetic clays, with very low natural abundance. In contrast to the smectite clays, the layers in anionic clays are composed of non-silicate materials. Moreover, the layers are positively charged, and hence anions rather than cations are found in the gallery region between the layers. As a result they do not undergo swelling in water and show completely different properties than smectite clays.
(3) Objects
A principal object of the present invention is to provide sorbent compositions suitable for diminishing SO.sub.x from flue gas streams particularly from coal-fired power plants. It is an object to provide highly reactive sorbent compositions with nano-scale particulates which give better SO.sub.x uptake in shorter time duration to overcome the low utilization of common oxide sorbents such as CaO and MgO due to mass transfer limitation and low reactivity towards SO.sub.2. A further object of the present invention is to employ modified and improved methods to disperse alkaline earth metal containing bases on clay supports. Further, it is an object of the present invention to provide composite sorbent compositions that are inexpensive to produce. These and other objects will become increasingly apparent by reference to the following description and the drawing.